In next generation wireless communication systems, multimedia data can be transmitted with high quality at a high speed by using limited radio resources. To achieve this, a spectral efficiency has to be maximized since a radio channel has a limited bandwidth. In addition, inter-symbol interference and frequency selective fading, which occur during high-speed transmission, have to be overcome.
In order to improve performance of the wireless communication system, a closed-loop transmission scheme using a channel condition between a base station (BS) and a user equipment (UE) has been introduced. An adaptive modulation and coding (AMC) scheme is used to improve link performance. In the AMC scheme, the BS controls a modulation and coding scheme (MCS) by using feedback of channel condition information.
In general, the UE informs the BS of a downlink channel condition in a well-known format, e.g., a channel quality indicator (CQI). The BS can receive the downlink channel condition from all UEs and perform frequency selective scheduling. To perform the frequency selective scheduling in uplink, the BS has to know an uplink channel condition as well.
A reference signal is used to measure the channel condition. The reference signal is known to both the BS and the UE and is also referred to as a pilot. An uplink reference signal has two types of signals, i.e., a demodulation reference signal and a sounding reference signal (SRS). The demodulation reference signal is used in channel estimation for data demodulation. The SRS is used in user scheduling irrespective of data transmission.
In addition to the SRS, a variety of data or uplink control information is transmitted on the uplink control channel. Examples of the uplink control signal are an acknowledgment (ACK)/negative-acknowledgement (NACK) signal used to perform hybrid automatic repeat request (HARM), a channel quality indicator (CQI) indicating downlink channel quality, a precoding matrix index (PMI), a rank indicator (RI), etc.
Uplink transmission is performed by the UE. Thus, it is important for the UE to have a low peak-to-average power ratio (PAPR) in order to decrease battery consumption. For this, a modulation scheme having single carrier characteristics can be selected in uplink transmission. The SRS is not related to uplink data or uplink control information. Therefore, when the SRS is transmitted simultaneously with other uplink control channel or uplink data, it is difficult to preserve the single carrier characteristics.
Accordingly, there is a need to effectively operate signals so that collision does not occur between a sounding signal and data or between the sounding signal and a control signal in order to preserve single carrier characteristics.